How can AI be used to design UI Interfaces?
If you see the use case, on higher level it seems to generate some visual output - the design but when seen at lower level, this design is output of some code. 

One way we can do it is to train a neural network that learns to generate code which can be seen as some form of organized text. So now can be treated as a text generation problem on which you can find a lot of literature.

pix2code is one implementation that uses this idea and even expands it. In this paper the authors took it to another level, they also used the visual part - the GUI and built an architecture that takes both the code and the GUI as input and learns to generate code. So eventually it would be capable of generating code when a simple GUI was given. 

Also there are some implementations where even the network could produce code even when a rough sketch was given.